Information records containing video, audio, and color information in the form of a very fine surface relief pattern in a plastic disc are known. The surface relief pattern is monitored by a playback stylus and the surface variations are reconstituted in electrical signal form and converted back to information suitable for display by a television receiver.
For instance, a capacitance video recording and playback system is disclosed in U.S. Pat. Nos. 3,842,194; 3,842,217; and 3,909,517 to Clemens. According to this system, disc replicas can be prepared having geometric variations in a spiral groove in the disc surface which correspond to capacitance variations representative of video signals. The discs are coated first with a thin conductive metal layer and then with a dielectric layer. A metal stylus completes the capacitor and, during playback, rides upon the dielectric coating and detects dimensional variations in the groove. These variations are reconstituted in electrical signal form and converted back to video information suitable for display by a television receiver. In the Clemens system the disc and the playback stylus are electrically conducting and a dielectric layer is between them. The need to provide two layers produced a system that was cumbersome and expensive and led to a search for a conductive molding composition from which a conductive molded disc could be made.
Since the relief pattern and the groove are of very small dimensions, e.g., there are 5,000-10,000 grooves per inch (1,968-3,937 grooves per centimeter), much research and expense have been required to learn how to put down metal and dielectric layers which conform to the relief pattern, are thin enough so that they do not fill the grooves, and yet are thick enough to form a coherent, continuous, abrasion resistant and pinhole free layer.
Fox et al in copending application Ser. No. 105,550, filed Dec. 20, 1979, a continuation of application Ser. No. 818,279 filed July 25, 1977, have described non-coated, conductive video disc replicas made by molding a plastic molding composition containing sufficient finely divided conductive particles so that the material has a bulk resistivity below about 500, and preferably below 100 ohm-cm at 900 megahertz. They disclose that conductive compression molded video discs could be made using a polyvinyl chloride homopolymer or a copolymer resin containing stabilizers, lubricants, and processing aids and conductive particles having a low bulk density in an amount sufficient to produce the required conductivity.
In the scale up of the Fox et al composition to produce commercial quantities, difficulties were encountered with the molding composition. When mixing large quantities of this material, high temperatures are generated due to the high shear and length of time required for dispersion of the ingredients, particularly large amounts of conductive particles. Excessive temperatures led to decomposition and high volatiles generation, as well as staining and excessive bleed out of the additives. Further, the melt viscosity of the molding composition was too high for compression molding on a large scale. In addition, the video disc replicas were rather brittle and tended to warp on storage under high temperature and high humidity conditions.
The Fox et al molding compositions were stiff and difficult to process and, thus, a search was undertaken to improve the processing of the molding composition. In U.S. Pat. No. 4,151,132 to Khanna, a conductive molding composition is described that has improved processability and produces video discs which are less brittle and have reduced staining or bleed out of additives. Bleed out is undesirable because it changes the surface characteristics of the discs, producing a high noise level. Khanna's solution was to mix small amounts of a large number of additives, including lubricants, stabilizers, and processing aids, into a mixture of vinyl chloride resins. These molding compositions are processable and moldable to form video discs of good quality. However, these discs are unsatisfactory from the standpoint of high temperature storage as the compositions have a comparatively low heat distortion temperature which results in permanent deformation or warpage and shrinkage of the disc when stored at elevated temperatures, e.g., above about 100.degree. F. (37.8.degree. C.).
Therefore, a search has continued to obtain conductive molding compositions which have good processability, are moldable to form conductive information discs of excellent playback characteristics, possess good thermal stability during compound processing and molding, provide good replication of submicron size surface relief patterns, and possess good dimensional stability on storage under various environmental conditions.